#1  
Old 06-05-2016, 09:48 AM
thegableguy's Avatar
thegableguy (Chris)
Registered User

thegableguy is offline
 
Join Date: Feb 2016
Location: NSW Central Coast, Australia
Posts: 337
Possible mount issues...?

Here's two shots from last night, one edited heavily and one relatively untouched from the resulting TIF from DSS. I shoot with a cheap DSLR through flattened ED80 on a NEQ6, unguided (with no PEC), 20 subs at 2 mins 30 seconds.

I used darks and too low an ISO so the luminance isn't really strong enough but the issue remains - the stars aren't just elongated, they're sliding back and forth resulting in these weird little lines with the brightest point in the middle.

Is this a characteristic of a mount that needs stripping down and regreasing? That's my best guess. On some 2-min subs the other night I noticed that every 3rd or 4th sub was blurred in one direction or the other; with these slightly longer subs it seems to be every 2nd or 3rd. I used all the subs for this image just to see what DSS would do with them; the fact that the strongest part of each star is in the middle suggests to me that the mount is going back and forth. Someone suggested a bit of grit in one of the gears could do that.

Thoughts?
Attached Thumbnails
Click for full-size image (Southern Pinwheel 160505-3.jpg)
207.1 KB52 views
Click for full-size image (Southern Pinwheel 160505-2.jpg)
195.4 KB45 views
Reply With Quote
  #2  
Old 06-05-2016, 10:48 AM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 6,384
This is known as Periodic Error and is completely normal, not much you can do about it without guiding OR replacing all of the mechanical components within the mount to a higher quality.

It takes about 7.5 minutes for the EQ6 to make one revolution of the worm gear, that means that the periodic error is over a 7.5 minute period. It changes from mount to mount (my EQ6 has a different PE than yours) but they're typically around the 10-30 arcsecond mark.

If your camera has 5 micron pixels and you image with a 80mm F/7.5 your image scale is 1.71"/pixel. If your mount has a PE peak to peak of 20", that is 10 in each direction that is a drift of about 2.5" per minute.

If you can get round stars at 2 minutes then I assume you have about 15" peak to peak periodic error which is reasonably good.
If you have gone over 2 minute exposures, you may just be pushing it too far for unguided.
Reply With Quote
  #3  
Old 06-05-2016, 10:51 AM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 6,384
One other though, when you image nearer to the south celestial pole, field rotation becomes more prominent. Just another thing to remember.
Reply With Quote
  #4  
Old 06-05-2016, 11:13 AM
luka's Avatar
luka
Unregistered User

luka is offline
 
Join Date: Apr 2007
Location: Perth, Australia
Posts: 1,117
As Colin said, this is typical PE. You can either fix things by having guiding or you can try improving things with PEC (PE correction) which EQ6 should be able to do (you can record PE without guiding, for example with a webcam on your ED80).
Reply With Quote
  #5  
Old 06-05-2016, 12:17 PM
thegableguy's Avatar
thegableguy (Chris)
Registered User

thegableguy is offline
 
Join Date: Feb 2016
Location: NSW Central Coast, Australia
Posts: 337
Thanks for that. Interesting.

Orright the whole PEC thing is entirely a mystery to me. For programming PEC does it have to be a live feed, or will a video file work? I don;t own a webcam, so wondering if I can use a DSLR - film for half an hour and then give it the video file.

Alternately my full frames can be tethered by USB with live view through Backyard Nikon; can they be used to program PEC? Would the error be big enough at 510mm for it to notice what was happening, or would I need to use a Barlow adapter?
Reply With Quote
  #6  
Old 06-05-2016, 12:25 PM
bojan's Avatar
bojan
amateur

bojan is offline
 
Join Date: Jul 2006
Location: Mt Waverley, VIC
Posts: 6,016
Besides PE, it could be a balance issue ... motors should always work against gravity, this way a small backlash doesn't matter much
Reply With Quote
  #7  
Old 06-05-2016, 01:38 PM
Somnium's Avatar
Somnium (Aidan)
Aidan

Somnium is offline
 
Join Date: Oct 2014
Location: Sydney
Posts: 1,669
Quote:
Originally Posted by bojan View Post
Besides PE, it could be a balance issue ... motors should always work against gravity, this way a small backlash doesn't matter much
yup could be backlash, wind/vibrations from the ground or PE ... that is my guess
Reply With Quote
  #8  
Old 06-05-2016, 02:18 PM
luka's Avatar
luka
Unregistered User

luka is offline
 
Join Date: Apr 2007
Location: Perth, Australia
Posts: 1,117
Regarding PE, I am not sure if you can use a DSLR.

Normally you would use your guiding camera (any webcam will do) and point it to a (bright) star. Then PHD analyses the star drift for an hour or so (for several worm rotations) and creates a file containing this information. Then something like PECprep (part of free EQMOD package) can be used to analyse this file and create a PEC file for EQASCOM. I am not sure how to upload it to EQ6 but it should not be too difficult (I believe?).

By the way, apart from a webcam you will need some sort of adapter to mount it to the scope.
Reply With Quote
  #9  
Old 06-05-2016, 03:55 PM
thegableguy's Avatar
thegableguy (Chris)
Registered User

thegableguy is offline
 
Join Date: Feb 2016
Location: NSW Central Coast, Australia
Posts: 337
- There was absolutely zero wind all night;
- I've got the counterweight set deliberately low so the pressure is always towards the ground;
- Though my crappy cheap T-ring has a little wiggle in it, I use the camera strap to "tie" the camera to my red dot finder so it can't move;
- The tripod itself is sitting on three 2.5kg weights which prevent it from sinking or moving in any direction (learnt that one the hard way);
- My polar alignment was pretty good from what I could tell;
- The movement is only in RA, but it's back and forth, not constantly in one direction, and it's not every exposure.

From what you're saying I think it's either PE or a need of re-greasing. Not owning a webcam or having any way of attaching one, I think I'm stuck with stripping it down as the first option and then will go from there. It's not a bad thing to do anyway, will be good to learn about how it works.

If it doesn't fix anything I'll have to decide between spending $1-200 on a webcam etc for PEC programming or $4-500 on a proper guiding system; likely I'll go the latter (if I can sneak it past my wife somehow).

Thanks everyone for your help!
Reply With Quote
  #10  
Old 06-05-2016, 04:04 PM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 6,384
Stripping the mount down and regreasing likely won't have any effect on the PE of the mount. This is simply just a mechanical issue that can only be resolved by either completely guiding or undergoing a complete rebuild (hyper tune) of its internals.
Reply With Quote
  #11  
Old 06-05-2016, 04:05 PM
Somnium's Avatar
Somnium (Aidan)
Aidan

Somnium is offline
 
Join Date: Oct 2014
Location: Sydney
Posts: 1,669
the counterweights should be set at a slight east bias so the gears are always driving rather than being pulled or bouncing back and forth.

If you need to borrow a guiding set up to train the pe I can lend you one.
Reply With Quote
  #12  
Old 06-05-2016, 05:05 PM
raymo
Registered User

raymo is offline
 
Join Date: Sep 2011
Location: margaret river, western australia
Posts: 4,990
For PE training all you need is a reticle eyepiece Chris, which most scope owners have. You don't even need a camera. 15 minute job. You don't need a computer or any software, just your scope. Page 38 of the Synscan manual. If you don't have the manual I strongly suggest that you download it from the SW website, print it out [46 pages] and make a book of it with a 65 cent plastic binder from a stationers. Just print out the relevant page for now. Lot of helpful stuff in there.
raymo

Last edited by raymo; 06-05-2016 at 05:38 PM. Reason: more text again
Reply With Quote
  #13  
Old 06-05-2016, 07:51 PM
thegableguy's Avatar
thegableguy (Chris)
Registered User

thegableguy is offline
 
Join Date: Feb 2016
Location: NSW Central Coast, Australia
Posts: 337
Oooh, really?

I don't have a reticle eyepiece, but I've got a red dot finder; would that work? Would be great if it was as simple as you say!
Reply With Quote
  #14  
Old 06-05-2016, 09:26 PM
raymo
Registered User

raymo is offline
 
Join Date: Sep 2011
Location: margaret river, western australia
Posts: 4,990
You must have a reticle eyepiece. An illuminated 12.5mm one can be had for about $69. All you do is get to the stage where the scope is tracking. Go into PEC training function, select the motor speed you want [depends on the field of view of your scope], centre a star in the reticle eyepiece, defocus the star so it is the size of the square in the centre of the eyepiece where the reticles cross, start the PEC function, and keep the star precisely filling the square using the right and left motor buttons until the mount tells you to stop. This training result will be retained for future sessions if you park the telescope before switching it off. If you switch off without parking the scope you will have to go through the procedure again next time you use the scope. Ignore any drift in Dec during the procedure, it obviously only deals with R.A.
As I said before, you really need to download the Synscan user manual.
Unfortunately a red dot finder cannot be used. You need the magnification
provided by a 12.5mm reticle EP, better still used with a barlow. It is simple, it takes about 15mins[2 revolutions of the drive gear].
raymo
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 07:02 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2020, Jelsoft Enterprises Ltd.
Advertisement
Astromechanics
Advertisement
Lunatico Astronomical
Advertisement
Celestron RASA
Advertisement
Star Adventurer
Advertisement
Testar
Advertisement
EQ8-R
Advertisement
Bintel
Advertisement
Astronomy and Electronics Centre
Advertisement